An Organic Rankine cycle (ORC) is named for its use of an organic, high molecular mass fluid with a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the water-steam phase change. The fluid allows Rankine cycle heat recovery from lower temperature sources such as biomass combustion, industrial waste heat, and waste heat from various small-scale heat engines, fuel cells or electric devices, geothermal heat, solar ponds, energy from combustion or decomposition of biodegradable materials. The low-temperature heat is converted into useful work that can itself be converted into electricity.
An idealized Clausius-Rankine cycle, also known as Rankine cycle, is characterized by an isentropic expansion, a heat dissipation at constant temperature, isentropic compression and an isobaric heating, which may be followed by a superheating. In a real Rankine cycle, the steps of expansion and compression are not exactly isentropic but the entropy increases slightly.
In the case of a “dry fluid”, the Rankine cycle can be improved by the use of a regenerator. In a temperature—entropy diagram, a dry-fluid is characterized by an overhanging mixed phase to vapour phase boundary. The dry fluid, which has not reached the two-phase state at the end of the expansion, has a temperature that is higher than the condensing temperature. The higher temperature fluid can be used to preheat the work fluid before it enters the evaporator.